1. Field of the Invention
This invention relates to a hydraulic closed circuit for use with a construction machine or an industrial vehicle, and more particularly to a safety device for a hydraulic closed circuit which is effective upon starting of a prime mover when the temperature is very low.
2. Description of the Prior Art
An exemplary one of conventional hydraulic closed circuits is shown in FIG. 8. Referring to FIG. 8, if a starter switch 2 is turned on when an operating lever 1 is at its neutral position, a battery 4 and a starter motor 5 are connected to each other via a relay 3, so that an engine 6 is started to rotate a variable displacement hydraulic pump 11 and a charge pump 12. Delivered oil from the charge pump 12 is introduced from the downstream side of a restrictor 13 into left and right cylinder chambers 16a and 16b of a tiltable cylinder 16 by way of a control valve 14 and a pair of pipe lines 15A and 15B. The cylinder chambers 16a and 16b thus have an equal internal pressure, and accordingly, a piston 16c of the tiltable cylinder 16 is positioned at its neutral position wherein the amount of tilting motion of the variable displacement hydraulic pump 11 is set to zero to restrict the delivery from the variable displacement hydraulic pump 11 to zero.
If the operating lever 1 is operated to change over the control valve 14 to a side thereof denoted by l, then the pressure on the upstream side of the restrictor 13 acts upon the cylinder chamber 16a while the pressure on the downstream side of the restrictor 13 acts upon the other cylinder chamber 16b, the piston 16c is displaced leftwardly in FIG. 8 by a distance corresponding to a difference between the pressures on the opposite sides of the restrictor 13, thereby setting an amount of tilting motion of the variable displacement hydraulic pump 11. Consequently, the variable displacement hydraulic pump 11 delivers pressure oil by an amount corresponding to the thus set amount of tilting motion into a main pipe line 17A to rotate a hydraulic motor 18 forwardly. On the contrary, if the control valve 14 is changed over to the other side denoted by m, then an amount of tilting motion of the variable displacement hydraulic pump 11 is set in the reverse direction, and consequently, pressure oil is delivered into another main pipe line 17B to rotate the hydraulic motor 18 reversely. 19 is crossover load relief valve and 20 is a flashing valve. A pair of check valves 21A and 21B are connected to the charge pump 12 by way of a pair of pipe lines 22a and 22b.
A position of the operating lever 1 is detected by a switch 7. When the operating lever 1 is, for example, at its neutral position, the switch 7 assumes its on position so that a coil 3C of the relay 3 is energized to close a switch 3S of the relay 3 to interconnect the starter switch 2 and the starter motor 5. Thus, only when the operating lever 1 is at its neutral position, starting of the engine 6 is permitted. To the contrary, if the operating lever 1 is operated, then the relay switch 3S is opened so that the engine 6 cannot be started. As a result, when the engine 6 is started, the variable displacement hydraulic pump 11 will not deliver pressure oil therefrom, thereby assuring the safety upon starting.
However, when the temperature is very low, the viscosity of operating oil is very high and accordingly the resistance of oil to the individual pipe lines is very high. This will raise the following problems.
In particular, if the engine 6 is started and then the control valve 14 is changed over to the l side, the pressures on the opposite sides of the restrictor 13 act upon the cylinder chambers 16a and 16b of the tilting cylinder after a predetermined delay in response due to very low temperature, so that the piston 16c is displaced in the leftward direction in FIG. 8 and the variable displacement hydraulic pump 11 delivers pressure oil to the main pipe line 17A. By the pressure oil, the hydraulic motor 18 is caused to start its rotation to drive a load, for example, to drive a vehicle to run. When the temperature is very low, however, the torque loss at the variable displacement hydraulic pump 11 and the charge pump 12 is so high and/or the viscosity of lubricating oil in the engine 6 itself is so high that the engine may stall.
If the operating lever 1 is returned to its neutral position immediately after such engine stall and then the starter switch 2 is turned on, then the engine 6 will be started because the relay switch 3S remains at its closed position. Since the pipe lines 15A and 15B are communicated with each other by way of the control valve 14 as the operating lever 1 is returned to its neutral position, the pressures within the hydraulic cylinders 16a and 16b are immediately made equal to each other. As a result, the piston 16c is returned from the leftwardly displaced position to the neutral position. In this instance, when oil discharged from the cylinder chamber 16a flows through the pipe line 15A, a high resistance acts upon the oil, so that it takes considerable time for the piston 16c to return to its neutral position.
Accordingly, there is a problem that, if the engine 6 is started before the piston 16c is returned to its neutral position, then pressure oil will be delivered from the variable displacement hydraulic pump 11 to activate the hydraulic motor 18 inadvertently although the operating lever 1 remains in its neutral position.